1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device and a structure formed therefrom, and more particularly to a method of fabricating an inductor and a structure formed therefrom.
2. Description of the Related Art
In integrated circuits, inductors are very important devices. The inductors are usually circular or rectangular spiral metal wires and widely used in different applications. For high-frequency devices, they require high performance of inductors which means that they have high quality factors. For example, in wireless communication, the quality factor (Q) of inductors should have about 60. The definition of quality factor is represented by the following formula:Q=ω0L/R  (1)
Wherein ω0 is the resonant angular frequency of the inductor, R is the resistance of the inductor and L is the inductance of the inductor.
From formula (a), when L is fixed, quality value will increase with respect to the increase of the resonant angular frequency or the decrease of the resistance, wherein the resistance is proportional to the square of the current density. Therefore, one method of enhancing the quality factor is to increase the cross-sectional area of the metal wires for reducing the current density therein. The method can reduce the resistance of the metal wires and increase the quality factor.
Therefore, in semiconductor devices the high quality factor can be achieved by increasing the width of the metal lines. However, if the width of the metal lines is too large, charges will accumulate at the corners of the metal lines and the current density therefore cannot be reduced. Accordingly, the quality factor of the inductor cannot be enhanced. Therefore, the quality factor generated from the prior art semiconductor process is about 10.
In addition, most of the inductors are formed under the protection layer of a chip and near to the substrate, usually smaller than 10 μm. Therefore, in the application of the high-frequency devices the substrate becomes a conductor and consumes most of energy. Accordingly, the performance of the inductors deteriorates.
Although prior art proposed a three-dimensional structure comprising metal lines, vias and metal lines, the inductor is still too close to the substrate. Moreover, because of the process restriction, the via pattern cannot be formed as that of the metal lines and just forms a plurality of plugs between the metal lines. Therefore, the quality factor can be improved. Accordingly, lots of attention is paid in improving the quality factor and performance of inductors.